Since Rudolf Diesel invented the basic diesel engine concept in 1893 constant improvements have been reached. The engine efficiency has increased from 5 to more than 40% today. Through decades of continues development work and efforts, reliability, drive ability and performance of this engine type have resulted in global acceptance and huge mass production. The number of diesel engines employed in vehicles, ships and power stations world wide have established a need for constant increased supply of fuel despite of all the above mentioned obtained engine improvements. The good fuel economy is one of the most important reasons for the achieved success measured as number of engines produced and sold compared to the diesel engines counterpart the spark ignition engine or Otto-cycle engine. Depending on engine size, fuel economy in diesel engines today is in the area of 20-40% more efficient than Otto-engines. Associated with the more and more wide spread use of internal combustion engines operated on fossil fuel, ecological and environmentally problems have aroused. Concerns for human health and global environmental changes due to the gases and solid particular matter associated with fuel combustion are of increasing international attention and concern. More specific, the amount of particulate matter, oxides of nitrogen and sulphur, is a problem for even modern diesel engines. Non-combusted fuel and carbon monoxide pose another negative environmental and human influence. To reduce those harmful emissions, improvements of various areas are under constant development. Among these main areas are: fuel injection systems, combustion chamber designs, turbocharger design, heat management systems like internal gas coolers, exhaust gas recycle, etc. Externally catalyst for various applications, absorber systems and particulate trap devises are used and improved. The quality of the diesel fuel is another way to improve combustion and reduce emissions. Better exhaust quality is obtained, to mention some, by desulphurisation, hydrogenation and by controlling fuel distillation properties at the refining process.
In spite of all the achieved improvements, the overall air quality still poses a huge problem on earth due to the number of vehicles running. The present fuel consumption and steady increase causes still more concerns for lack of sustainability for future energy supply. There are obviously three main problems to attend to:                Future energy supply security        Air quality in urban areas (toxic components)        Global air changes (greenhouse gases)        
Carbondioxide is a greenhouse gas believed to increase global atmospheric temperature. Limitations of the amount of carbondioxide produced and emitted associated with combustion in internal combustion engines are closely linked to the efficiency of the engine. Generally speaking, diesel engines operate with 30% better fuel efficiency compared to spark assisted Otto-engines. The 30% better fuel efficiency corresponds roughly to 30% less emitted carbondioxide. Natural gas and methanol are both Otto-engine fuels. In the above description a method to improve efficiency for methanol combustion in internal combustion engines by reformulation on board a vehicle, gaining diesel efficiency is outlined.
Generating methanol from natural gas is today an established technology worldwide. More than 25 MMT is produced annually. The methanol production capacity has increased every year for more than a decade now. Known reserves of natural gas recourses, relevant for further production and expansion counts for at least an equivalent energy mass compared to proven oil reserves. While oil and oil deviated fossil fuels are the main source of energy used for transportation today, natural gas and natural gas deviated products counts for a limited use, directly as compressed natural gas (CNG) and liquefied natural gas (LNG) or indirectly in forms of and liquefied deviated products like methanol, ether and Fischer Trops (FT) type products. One of the possible strategies for energy production is more efficient use of the recourses. Another coherent possibility is to change the ratio between oil and gas deviated products used towards the gas technologies. A change in the direction of more efficient utilisation of natural gas based processes combined with more efficient utilisation of the generated fuels used in the transportation sector is a key issue in the present invention. Increasing efficiency could be achieved by increasing production capacity of new industrial production plants (economy of scale) as an example. This combined with development of high efficient use of the produced deviates makes the overall efficiency increase possible. More specifically methanol is today used as a fuel in Otto-engines directly or mixed with gasoline neat or chemically reacted as MTBE as octane improves. In all cases the fuels are suited for Otto engines and used in Otto engines. If the produced fuel could be used directly or indirectly in a diesel cycle engine, increased fuel efficiency of 20-40% would be expected. Many ways have been suggested to achieve this.
To mention one close related way, it has been suggested to convert methanol to dimethylether (DME) and use this as a diesel fuel (U.S. Pat. No. 5,906,664). DME can also be produced in a direct process (Topsoe patent DK 171, 707) and save both investments in plant construction and also in operating costs. DME has shown to be a superior diesel fuel with good combustion characteristic. No particulate is formed and lower emissions of NOx are achieved with no penalty in fuel efficiency. Fuel efficiency is corresponding to diesel fuel based upon energy equivalent mass. However, infrastructure and logistic are more expensive than traditional requirements for gasoline and diesel.
In order to achieve the good efficiency of DME and to avoid the more expensive logistic, methanol is produced, distributed and used as the basic energy carrier. Then methanol is converted on board a vehicle to DME. Methanol conversion is performed by means of a catalytic converter. Catalyst used for the dehydration process is known and applied for in the aerosol industry. The catalytic converter is heated by surplus heat from the exhaust gas using a heat exchanger. The obtained fuel is suitable as fuel for direct and indirect diesel injection engines provided with a heater on the air intake.
Storing a small amount of the generated fuel makes instant engine start possible. Further advantages using the generated fuel composition are the CO-produced water. This water reduces formation of oxides of nitrogen (NOx) due to a reduction in combustion temperature to even lower level than operation with neat DME. Small amounts of unconverted methanol, which is present due to thermodynamic reasons, is acting as CO-solvent for the DME/water mixtures securing that only one single phase of stable fuel is obtained with a very low freezing point.
The present invention relates to a method by which the goal of high efficiency and low emissions can be reached by modifying an Otto-fuel with low fuel efficiency to a diesel fuel with high fuel efficiency. More specifically, methanol, which is a fuel for Otto-engine with high Octane number (115) and low Cetane number (5), is converted to preliminarily dimethylether (DME) with low Octane number and high Cetane number (>55) and water by a catalytic dehydration process. The obtained fuel comprises some unconverted methanol, generated ether as the main component and associated water. This fuel composition is used directly as fuel in a diesel engine. The generated dimethylether has a high Cetane number (>55) and is known to be a superior diesel fuel. From two mole of methanol, one mole of DME and one mole of water are formed. The chemical reaction can be written as follows:Methanol dehydration 2CH3OH→CH3OCH3+H2O
Equilibrium limitations prevent a 100% conversion of methanol to DME and water. This means that methanol conversion or dehydration to ether and water always leaves some unconverted methanol unless a separation or distillation process is applied. However, experimental engine test has concluded that separation of the produced fuel is not required. It has been found that only preheating of the inlet air combined with the catalytic dehydration process is necessary to obtain smooth combustion in a diesel engine.
The fuel composition obtained by dehydration of dry methanol then consists of a constant molecular ratio between DME and water, which is 1:1. Expressed as mass ratio, the ratio of DME and water will be 2.56. Depending on methanol dehydration degree the amount of methanol in the DME/water produced will counts for 0 at 100%. 0% at 100% conversion (theoretically) and 100% at zero conversion. This explanation is shown graphically as a trapeze diagram in FIG. 1 attached.
Modern industrial methanol production plants today are equipped with high quality distillation utilities in order to make high quality chemical grade methanol. If a more simple separation is applied, savings in investment and operating can be obtained. A cheaper fuel grade methanol is then obtained. With the trade-off that this crude or technical grade methanol contains some ethanol, higher alcohol and water up to 20 wt %. However, these alcohol are dehydrated as well as methanol is dehydrated yielding good diesel quality property. The chemical reaction for generating diethylether from ethanol can be written as follows:Ethanol dehydration 2C2H5OH→CH3CH2OCH2CH3+H2O
The formed diethylether has a low self-ignition temperature (160° C.), which is even lower than DME (237° C.). Higher alcohol is dehydrated likewise to ethers and water. The molecular ratio between ether and water formed are always 1:1 increasing molecular weight of the ether increases the weight ratio between the ether and the water. Methanol dehydration results in a mass ratio between formed dimethylether and water corresponding to 2.6. Ethanol dehydration yields a mass ratio of 4.1 between formed diethylether and water. Evaluation of test fuel containing diethylether is given in Table 6 described later. Minor concentrations of higher alcohol are converted to ether according to the above-described chemical equation.Alcohol dehydration: 2CnH(2n+2)O→C2nH(4n+2)+H2O
To evaluate the influence of water, experimental studies have confirmed that very high concentrations can be tolerated on the condition that sufficient high air inlet temperature are available according to the present invention. Typically, water is closely associated with alcohol production. The price for separation of the alcohol from the water in mixtures depends upon the specific production unit. Usually, this separation cost raise exponentially with the demands for purity. A trade-off in this situation with the fuel price, the price for moving around with the fuel containing water and the fuel efficiency obtained in the engine have to be determined in all situations. Generally speaking more water can be accepted produced and used close to the engine.
Detailed results of experimental evaluation on aqueous fuels are given in Table 7 described later.